Piercing method, production method for structure having hole, and structure having hole

ABSTRACT

A piercing method using a hot-press includes: a piercing process of piercing a workpiece with a piercing edge; a concave portion forming process of forming a concave portions around a hole formed in the workpiece by the piercing process to enlarge a diameter of the hole; and a cooling process of cooling the workpiece.

This application claims priority from Japanese Patent Application No. 2012-156393, filed on Jul. 12, 2012, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a piercing method using a hot-press which presses a heated structure and then cools the latter to perform quenching, a method of fabricating a structure having a hole, and the structure having the hole.

RELATED ART

A method for production of a structure using a hot-press has been known. In the hot-press, the structure is first heated to about 1000° C., and the heated structure is subjected to a press process. After that, the structure is cooled and quenched to transform a steel plate structure from an austenite structure obtained at heating to a martensite structure, so that the structure subjected to the press working is formed to have high hardness.

Hitherto, Japanese Unexamined Patent Publication 2005-248253 (Patent Document 1) discloses a technology of extending a lifespan of a tool for performing boring or trimming when the structure, such as a vehicular body component, is fabricated using the hot-press. In the technology disclosed in Patent Document 1, when the heated structure is form-shaped by a mold, the structure is worked simultaneously by the tool. According to the technology, since the working of the structure using the tool is performed before the structure is formed to have the high hardness through the quenching by cooling, the burden on the tool can be reduced, thereby extending the lifespan of the tool.

However, even by using the technology disclosed in Patent Document 1, it is not easy to form a hole in the structure.

FIG. 5 is a diagram illustrating a piercing method using the hot-press according to the related art and its problem, in which FIG. 5( a) shows a piercing process, and FIG. 5( b) shows the problem thereof.

As illustrated in FIG. 5( a), when the press forming is carried out by putting a heated structure W101 on a lower die 202 and moving an upper die 201 to a bottom dead center, the structure W101 is subjected to the piercing working by a piercing edge 301 which is formed integrally with the upper die 201. After the structure W101 is formed with a hole H401 by the piercing edge 301, the upper die 201 is necessarily held at the bottom dead center to prevent the structure W101 from being deformed due to contraction caused by degradation in temperature.

However, in the state in which the upper die 201 is held at the bottom dead center, that is, the piercing edge 301 is inserted in the hole H401, if the structure W101 is subjected to the quenching by cooling, the structure W101 is contracted, so that a diameter of the hole H401 is decreased. As illustrated in FIG. 5( b), the piercing edge 301 is restrained by the contracted hole H401. For this reason, it is difficult to separate the piercing edge 301 restrained by the hole H401 with contracted diameter from the structure W101.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem, and an object of the present invention is to provide a piercing method of easily carrying out piercing working using a hot-press, a method of fabricating a structure having a hole, and the structure having a hole.

(1): A piercing method using a hot-press includes: a piercing process (for example, the cutting process S4 which will be described later) of piercing a structure (for example, the workpiece W which will be described later) with a cutting edge (for example, the piercing edge 24 a which will be described later); a concave/convex forming process (for example, the concave portion forming process S5 which will be described later) of forming a concave/convex portion (for example, the concave portions C which will be described later) around a hole (for example, the hole H which will be described later) formed in the structure by the piercing process to enlarge a diameter of the hole; and a cooling process (for example, the cooling process S6 which will be described later) of cooling the structure.

According to the aspect (1), since the diameter of the hole is enlarged by the slightly small amount by forming the concave/convex portion around the hole, the hole is not contracted smaller than a diameter of the cutting edge, even though the structure cooled in the cooling process is contracted. Accordingly, even though the cooling process is performed in the state in which the cutting edge is inserted in the hole after the piercing working is carried out, the cutting edge is not restrained in the hole, so that the cutting edge can be easily separated from the structure having high hardness. Accordingly, the piercing working can be easily carried out in the hot-press.

(2): The piercing method according to the aspect (1), wherein an enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is larger than a contraction diameter amount of the hole by the cooling process.

According to the aspect (2), since the enlarged diameter amount of the hole by the concave/convex portion is larger than the contracted diameter amount by the cooling process, the enlarged diameter of the hole can be maintained even though the structure cooled by the cooling process is contracted.

(3): The piercing method according to the aspect (1) or the aspect (2), wherein the enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is large as a direction of a radially contracted diameter amount of the hole by the cooling process is large.

According to the aspect (3), the enlarged diameter amount of the hole by the concave/convex portion is large as the direction of the radially contracted diameter amount by the cooling process is large. Therefore, in the case where the radially contracted diameter amount by the cooling process is different, the shape of the hole is not significantly deformed from the shape obtained by the piercing process.

(4): A method of fabricating a structure (for example, the workpiece W which will be described later) having a hole (for example, the hole H which will be described later) using a hot-press, includes: a heating process (for example, the heating process S1 which will be described later) of heating the structure; a piercing process (for example, the cutting process S4 which will be described later) of piercing the structure with a cutting edge (for example, the piercing edge 24 a which will be described later); a concave/convex forming process (for example, the concave portion forming process S5 which will be described later) of forming a concave/convex portion (for example, the concave portions C which will be described later) around a hole (for example, the hole H which will be described later) formed in the structure by the piercing process to enlarge a diameter of the hole; and a cooling process (for example, the cooling process S6 which will be described later) of cooling the structure while the cutting edge is inserted in the hole, wherein an enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is larger than a contraction diameter amount of the hole by the cooling process.

According to the aspect (4), since the concave/convex portion having the enlarged diameter amount of the hole larger than the contracted diameter amount by the cooling process is formed around the hole, the diameter of the hole is enlarged by the slight small amount. Therefore, the hole can be maintained in the enlarged diameter, even though the structure cooled in the cooling process is contracted. Accordingly, even though the cooling process is performed in the state in which the cutting edge is inserted in the hole after the piercing working is carried out, the cutting edge is not restrained in the hole having the diameter of the enlarged diameter, so that the cutting edge can be easily separated from the structure having high hardness. Accordingly, the piercing working can be easily carried out in the hot-press.

(5): A structure (for example, the workpiece W which will be described later) having a hole (for example, the hole H which will be described later) which is fabricated by a hot-press, includes: a hole formed by a piercing process using a cutting edge (for example, the piercing edge 24 a which will be described later); and a concave/convex portion (for example, the concave portions C which will be described later) formed around the hole to enlarge a diameter of the hole, wherein the hole and the concave/convex portion are formed after a heating process of heating the structure in the hot-press and before a cooling process (for example, the cooling process S6 which will be described later) of cooling the structure.

According to the aspect (5), the same effect as that of the aspect (1) is shown.

(6): A structure (for example, the workpiece W which will be described later) having a hole (for example, the hole H which will be described later) which is fabricated by a hot-press, includes: a hole formed by a piercing process using a cutting edge (for example, the piercing edge 24 a which will be described later) after a heating process (for example, the heating process Si which will be described later) of heating the structure in the hot-press, the hole being subjected to a cooling process (for example, the cooling process S6 which will be described later) of cooling the structure in a state in which the cutting edge is inserted in the hole; and a concave/convex portion (for example, the concave portions C which will be described later) formed around the hole after the piercing process, the concave/convex enlarging a diameter of the hole so that an enlarged diameter amount of the hole is larger than a contracted diameter amount of the hole by the cooling process.

According to the aspect (6), the same effect as that of the aspect (4) is shown.

As stated above, the present invention provides the piercing method capable of easily performing piercing using the hot-press, the method of fabricating the structure having the hole, and the structure having the hole.

Other aspects and advantages of the invention will be apparent from the following description, the drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating each process of a hot-press forming method according to an embodiment of the present invention.

FIG. 2 is a flowchart of the hot-press forming method according to the embodiment.

FIG. 3 is an enlarged view around a hole of a workpiece formed with concave portions according to the embodiment.

FIG. 4 is an enlarged view around the hole of the workpiece fabricated by the hot-press forming method according to the embodiment.

FIG. 5 is a diagram illustrating a piercing method using the hot-press according to the related art and its problem.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

An embodiment of the present invention will now be described with reference to the accompanying drawings. First, a mold 1 for use in a hot-press forming method will be described. FIG. 1 is a diagram illustrating each process of the hot-press forming method according to the embodiment of the present invention, in which FIG. 1( a) shows a placing process, FIG. 1( b) shows a form shaping process, FIG. 1( c) shows a cutting process, and FIG. 1( d) shows a concave portion forming process.

A workpiece W formed by the hot-press forming method is a structure, such as a vehicular body component, made of a high-tensile steel plate. If the workpiece is subjected to press forming by the hot-press forming method, it is fabricated as a structure having high hardness.

The mold 1 for use in the hot-press forming method includes, as illustrated in FIG. 1, an upper die 2 and a lower die 3. The upper die 2 is a movable type which is moved down when the press is carried out, while the lower die 3 is s stationary type.

The upper die 2 includes a pad 21 having an upper forming surface 21 a for performing form shaping of substantially truncated cone shape on the workpiece W in cooperation with a lower forming surface 3 a of the lower die 3 which will be described later. The pad 21 is attached to the upper die 2, with an elastic body 22 being interposed therebetween. The elastic body 22 is contracted after a form shaping process which will be described later, so that the pad 21 approaches the upper die 2.

The upper die 2 has an upper trimming portion 23 in a surrounding region of the pad 21 which performs the form shaping. The pad 21 is provided with a piercing portion 24 and a score punching portion 25 at a substantially center portion thereof.

The upper trimming portion 23 is disposed in the surrounding region of the pad 21, and is integrally attached to the upper die 2. The upper trimming portion 23 has an upper trimming edge 23 a for performing a cutting process on an end portion of the workpiece W.

The upper trimming edge 23 a cuts the end portion of the workpiece W in cooperation with the lower trimming edge 31 a.

The piercing portion 24 is disposed at the center of a hole 21 b provided in the substantially center portion of the pad 21 m, and is integrally attached to the upper die 2. The piercing portion 24 has a piercing edge 24 a for performing the piercing working on the workpiece W to have the hole H (see FIG. 3) of a true circular shape having a diameter of A. The piercing edge 24 a has a circular shape in cross section, and forms the hole H in the workpiece W when the piercing edge is accommodated in an aperture 33 of the lower die 3 which will be described later.

The score punching portion 25 is disposed in a substantially annular type around the piercing portion 24 with respect to the hole 21 b provided in the substantially center portion of the pad 21. The score punching portion 25 has a score punch 25 a for forming concave portions C, which will be described later, on the workpiece W. The score punch 25 a may be a semi-spherical convex portion by placing a dent around the hole H of the workpiece W.

The upper trimming portion 23 and the piercing portion 24 are configured so that the upper trimming edge 23 a and the piercing edge 24 a cut the workpiece W, before the score punch 25 a forms the concave portions C on the workpiece W. The upper trimming portion 23 and the piercing portion 24 may be configured so that the cutting by the upper trimming edge 23 a and the cutting by the piercing edge 24 a are performed at a substantially same timing. That is, the upper trimming portion 23 protrudes downward rather than the piercing portion 24, but may be disposed at a position in which the cutting of the workpiece W by the upper trimming edge 23 a is performed at the substantially same timing as the cutting of the workpiece W by the piercing edge 24 a.

The piercing portion 24 protrudes downward rather than the score punching portion 25 to perform the cutting of the workpiece W by the piercing edge 24 a, before the concave portions C are formed on the workpiece W by the score punch 25 a.

The core punching portion 25 is disposed to form the concave portions C around the hole H of the workpiece W at a timing when the upper die 2 reaches the bottom dead center, after the workpiece W is cut by the piercing edge 24 a to form the hole H.

The lower die 3 has the lower forming surface 3 a fixed to a bottom surface to performing the form shaping on the workpiece W in cooperation with the upper forming surface 21 a of the pad 21.

The lower die 3 has a lower trimming portion 31 below the pad 21, a tray 32 below the upper trimming portion 23, the aperture 33 below the piercing portion 24, and concave portions 34 below the score punching portion 25.

The lower trimming portion 31 is disposed inside the mold 1 rather than the upper trimming portion 23, and is integrally attached to the lower die 3. The lower trimming portion 31 has a lower trimming edge 31 a for cutting the end portion of the workpiece W in cooperation with the upper trimming edge 23 a, with the workpiece W being interposed between the lower and upper trimming edges.

The tray 32 receives a cut piece E fallen from the workpiece W which is interposed between the upper and lower trimming edges 23 a and 31 a and is cut by them.

A hole piece D fallen from the workpiece W which is pierced by the piercing edge 24 a passes through the aperture 33.

The concave portion 34 may be a semi-spherical dent, and may be formed around the aperture 33 of the lower forming surface 3 a of the lower die 3 by pressing a front end of the score punch 25 a when the score punch 25 a forms the concave portions C.

Since the upper die 2 is provided with the score punch 25 a, the lower forming surface 3 a of the lower die 3 is provided with the concave portions 34, and a protrusion is not provided on the lower forming surface 3 a, the workpiece W placed on the concave portions 34 is held, without being floated on the lower forming surface 3 a of the lower die 3. For this reason, in the form shaping process S3 or the cutting process S4, the product precision of the workpiece W is not deteriorated, since misalignment does not occur due to the floating of the workpiece W.

Next, the hot-press forming method according to the embodiment will be described. FIG. 2 is a flowchart of the hot-press forming method according to the embodiment.

(Heating Process S1)

In the hot-press forming method, first, the workpiece W is heated to about 1000° C.

(Placing Process S2)

As illustrated in FIG. 1( a), the heated workpiece W is placed at a predetermined position on the lower forming surface 3 a of the lower die 3 in a mold opening state in which the upper die 2 is moved up to a top dead center.

(Form Shaping Process S3)

As the upper die 2 is moved down from the top dead center, and as illustrated in (b) of FIG. 1, the pad 21 of the upper die 2 abuts against the workpiece W placed on the lower die 3, and then the workpiece W is pressed by the pad 21 and the lower die 3, thereby performing the form shaping on the workpiece W with the upper forming surface 21 a and the lower forming surface 3 a.

(Cutting Process S4)

When the upper die 2 is further moved down, as illustrated in FIG. 1( c), in the state in which the abutment of the pad 21 against the workpiece W is maintained, the elastic body 22 is contracted, and then the upper trimming edge 23 a of the upper trimming portion 23 of the upper die 2 comes into contact with the workpiece W, thereby performing the cutting process to cut the end portion of the workpiece W between the upper and lower trimming edges 23 a and 31 a. At nearly the same time, the piercing edge 24 a of the piercing portion 24 of the upper die 2 abuts against the workpiece W, and the piercing working is performed to form the hole having the diameter of A in the workpiece W by the piercing edge 24 a.

(Concave Portion Forming Process S5)

When the upper die 2 is further moved to the bottom dead center, as illustrated in (d) of FIG. 1, the score punch 25 a of the score punching portion 25 of the upper die 2 abuts against the workpiece W, and then a portion of the workpiece W is pressed against the concave portions 34 of the lower die 3 by the score punch 25 a to form the concave portions C.

The workpiece W formed with the concave portions C will now be described. FIG. 3 is an enlarged view around the hole H of the workpiece W formed with the concave portions C according to the embodiment, in which FIG. 3( a) is a perspective view, and FIG. 3( b) is a cross-sectional view. As illustrated in FIG. 3, the concave portions C are placed around the hole H of the workpiece W which is cut by the piercing edge 24 a.

The concave portions C enlarge the hole H to have a diameter of B, which is indicated by a solid line, by a slightly small amount by pulling a thickness around the hole H having the diameter of A, which is indicated by a broken line, of the workpiece W to the concave portions C at an outer diameter side of the hole H, and the enlarged diameter amount of the hole H by the concave portions C is set to be larger than a refracted diameter amount by the cooling process S6. Further, it is preferable that much concave portions C are formed, as the radially contracted diameter amount by the cooling process S6 is large. That is, in a case where the radially contracted diameter amount by the cooling process S6 is different, for example, when a position of the hole H in the workpiece W is not an approximately center portion, but is near to a bent portion of the workpiece W, the enlarged diameter amount of the hole H by the concave portions C is preferably set to be larger, as the direction of the radially contracted diameter amount by the cooling process S6 is large. According to the shape of the hole H having the diameter of B, of which the diameter is enlarged by the concave portions C, the radially enlarged diameter amount is increased in the region where many concave portions C are formed. The concave portions C are disposed so that the hole H has a wanted shape (e.g., a true circular shape which is larger than the true circle having the diameter of A when it is formed in the cutting process S4, and is not significantly deformed) after the cooling process S6.

(Cooling Process S6)

Returning to the description about the process of the hot-press forming method illustrated in FIG. 2, after the working of the concave portions C, the workpiece W is cooled together with the mold 1 in the state in which the upper die 2 is held in the bottom dead center, to prevent the workpiece W from being deformed due to the contraction caused by the degradation in temperature. If the workpiece W is contracted by the cooling, the hole H having the diameter of B, of which the diameter is enlarged according to the radially enlarged diameter amount by the concave portions C, is contracted in diameter according to the radially contracted diameter amount, and thus is formed in a substantially true circle having at least a diameter of C (see FIG. 4( a)) larger than the diameter of A.

In this instance, since the hole H is enlarged in diameter by the concave portions C against the radially contracted diameter amount by the cooling process S6 (diameter C>diameter A), the piercing edge 24 a inserted in the hole H of the workpiece W is not restrained even when the hole H is contracted in diameter by the contraction of the workpiece W, so that the piercing edge can be easily separated from the workpiece W.

Then, when the upper die 2 is moved to the top dead center to open the mold, the piercing edge 24 a is extracted from the hole H having the diameter of C without being brought into contact with the hole, and the workpiece W is withdrawn from the mold 1.

Now, the workpiece W fabricated by the hot-press forming method will be described. FIG. 4( a) is an enlarged view around the hole H of the workpiece W fabricated by the hot-press forming method according to the embodiment. As illustrated in FIG. 4( a), the work W having the hole H which is fabricated by the hot-press forming method includes the hole H which is formed by the piercing working using the piercing edge 24 a in the cutting process S4 after the heating process S1, and is subjected to the cooling process S6 to cool the workpiece W in the state in which the piercing edge 24 a is inserted in the hole H, and the concave portions C which are formed around the hole H in the concave portion forming process S5 after the piercing working, and enlarge the diameter of the hole by the enlarged diameter amount larger than the contracted diameter amount by the cooling process S6.

As illustrated in FIG. 4( a), supposing that the diameter of the hole subjected to the piercing working by the piercing edge 24 a and the outer diameter of the piercing edge 24 a which is substantially equal to the diameter are the diameter of A indicated by the broken line, the enlarged diameter of the hole which is enlarged in diameter by the concave portions C is the diameter of B indicated by a double-dotted line, and the contracted diameter of the hole which is contracted in diameter by the cooling is the diameter of C indicated by the solid line, the relationship of diameter B>diameter C>diameter A is established. The relationship of diameter B>diameter C>diameter A causes the piercing edge 24 a having the outer diameter of A to be easily withdrawn from the hole H having the diameter of C.

The following effects are shown by the hot-press forming method according to the embodiment.

(1) Since the diameter of the hole H is enlarged by the slightly small amount by forming the concave portions C around the hole H, the enlarged diameter amount of the hole H being larger than the retracted diameter amount by the cooling process S6, the diameter of the hole H can be maintained in the enlarged state (diameter C>diameter A), even though the workpiece W cooled in the cooling process S6 is contracted. Accordingly, even though the hole H inserted with the piercing edge 24 a is subjected to the cooling process S6 after the piercing working is carried out, the piercing edge 24 a is not restrained in the hole H which is enlarged in diameter, so that the piercing edge 24 a can be easily separated from the workpiece W having the high hardness. Accordingly, the piercing working can be easily carried out in the hot-press forming method.

(2) Since the enlarged diameter amount of the hole H by the concave portions C is larger than the contracted diameter amount by the cooling process S6, the enlarged diameter of the hole H can be maintained even though the workpiece W cooled by the cooling process S6 is contracted.

(3) The enlarged diameter amount of the hole H by the concave portions C is large as the direction of the radially contracted diameter amount by the cooling process S6 is large. Therefore, in the case where the radially contracted diameter amount by the cooling process S6 is different, the shape of the hole H is not significantly deformed from the shape obtained by the cutting process S4.

The invention is not limited to the above embodiment, and can be varied or modified without departing from the spirit or scope defined by the appended claims.

In the embodiment, the concave portions C are formed around the hole H by the score punch 25 a, but the present invention is not limited thereto. If the hole H is enlarged in diameter, a convex portion or a concave/convex portion may be formed.

In the embodiment, much concave portions C are formed as the direction of the radially contracted diameter amount by the cooling process S6 is large, but the present invention is not limited thereto. The concave portions C is preferably formed to be large (make a machining amount or a machining area large), as illustrated in FIG. 4( b), as the direction of the radially contracted diameter amount by the cooling process S6 is large. Further, much concave portions C may be simultaneously formed. That is, it is desirable that the enlarged diameter amount of the H by the concave portions C is set to be larger, as the direction of the radially contracted diameter amount by the cooling process S6 is large.

In the embodiment, much concave portions C are formed as the direction of the radially contracted diameter amount by the cooling process S6 is large, but the present invention is not limited thereto. The concave portions C may be placed in plural at regular intervals at an equal radius position from the center of the hole H around the hole H, as illustrated in FIG. 4( c), or may be formed in a circular shape to enclose the hole H, as illustrated in FIG. 4( d). In this instance, the diameter of the hole can be always maintained in a true circular shape in each process of the hot-press forming method, and the shape of the hole H after the quenching can be easily formed in a wanted true circular shape.

In the embodiment, the timing of cutting the end portion of the workpiece W by the upper and lower trimming edges 23 a and 31 a and the timing of piercing the workpiece W by the piercing edge 24 a are almost equal, but the present invention is not limited thereto. Both working may be performed at a different timing.

Further, in the embodiment, the form shaping and the cutting are performed simultaneously, but the present invention is not limited thereto. The forming may be forming such as drawing forming, except for the form shaping, or the forming and the cutting may not be performed simultaneously. 

1. A piercing method using a hot-press, comprising: a piercing process of piercing a structure with a cutting edge; a concave/convex forming process of forming a concave/convex portion around a hole formed in the structure by the piercing process to enlarge a diameter of the hole; and a cooling process of cooling the structure.
 2. The piercing method according to claim 1, wherein an enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is larger than a contraction diameter amount of the hole by the cooling process.
 3. The piercing method according to claim 1, wherein the enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is large as a direction of a radially contracted diameter amount of the hole by the cooling process is large.
 4. A method of fabricating a structure having a hole using a hot-press, comprising: a heating process of heating the structure; a piercing process of piercing the structure with a cutting edge; a concave/convex forming process of forming a concave/convex portion around a hole formed in the structure by the piercing process to enlarge a diameter of the hole; and a cooling process of cooling the structure while the cutting edge is inserted in the hole, wherein an enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is larger than a contraction diameter amount of the hole by the cooling process.
 5. A structure having a hole which is fabricated by a hot-press, comprising: a hole formed by a piercing process using a cutting edge; and a concave/convex portion formed around the hole to enlarge a diameter of the hole, wherein the hole and the concave/convex portion are formed after a heating process of heating the structure in the hot-press and before a cooling process of cooling the structure.
 6. The piercing method according to claim 1, wherein the enlarged diameter amount of the hole by the concave/convex portion which is formed by the concave/convex forming process is large as a direction of a radially contracted diameter amount of the hole by the cooling process is large. 